Free-machining aluminum alloy extruded material with reduced surface roughness and excellent productivity

ABSTRACT

To obtain an Al—Mg—Si based aluminum alloy extruded material with a smooth surface and no burning without inhibiting the productivity. An aluminum alloy billet includes: Si: 2.0 to 6.0% by mass; Mg: 0.3 to 1.2% by mass; and Ti: 0.01 to 0.2% by mass, a Fe content being restricted to 0.2% or less by mass, with the balance being Al and inevitable impurities. The aluminum alloy billet is subjected to a homogenization treatment by keeping at 500 to 550° C. for 4 to 15 hours. The billet is forcibly cooled to 250° C. or lower at an average cooling rate of 50° C./hr or higher. Then, the billet is subjected to hot-extruding at an extrusion rate of 3 to 10 m/min by being heating at 450 to 500° C. The extruded material is forcibly cooled at an average cooling rate of 50° C./sec or higher and then subjected to an aging treatment. The extruded material can be manufactured that has its surface having a ten-point average roughness Rz of 80 μm or less.

TECHNICAL FIELD

The present invention relates to an Al—Mg—Si based aluminum alloyextruded material that is suitable for use in mechanical parts and thelike, requiring many machining processes during manufacturing proceduresand that has high strength and excellent machinability, and also to amanufacturing method thereof.

BACKGROUND ART

Patent Documents 1 to 4 disclose the Al—Mg—Si based aluminum alloyextruded materials for machining. To improve the machinability in thesealuminum alloy extruded materials for machining, 1.5% or more by mass ofSi is added, and a large amount of Si crystallized grains (Si phase),which are second-phase hard particles, are distributed in a matrix.

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: JP 9-249931 A-   Patent Document 2: JP 10-8175 A-   Patent Document 3: JP 2002-47525 A-   Patent Document 4: JP 2003-147468 A

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The Al—Mg—Si based aluminum alloy for machining crystallizes into Si andMg₂Si during the solidification process, and also crystallizes into aneedle-like β-AlFeSi-based compound (β-AlFeSi phase) made of Fe asinevitable impurities, and Al and Si. FIG. 1 shows a micrograph of abillet before a homogenization treatment. Strip-shaped Si phases (ingray) are connected in a net shape, in which Mg₂Si phases (in black) aredistributed as dots, while needle-like β-AlFeSi phases (in white) areformed along the Si phases. Extruding the Al—Mg—Si based aluminum alloybillet poses a problem in which burning (pickup) might occur in extrudedmaterials, degrading the smoothness of the surface of the extrudedmaterial.

The occurrence of burning in the Al—Mg—Si based aluminum alloy extrudedmaterial is based on the following reasons. The strip-shaped Si phasesexisting in the billet before extruding cause a eutectic reaction withan Al phase and a Mg₂Si phase due to the deformation of material byextruding and the heat generation during the process resulting from thefriction between the material and a die-bearing portion, thereby causinglocal melting. A shearing force upon the extruded material when itpasses through the die-bearing portion makes the material of the surfaceof the extruded material (cells surrounded by the Si phases) fall offstarting at the melting point, causing burning of the extruded material.

Further, the needle-like β-AlFeSi phases existing in the billet beforeextruding cause a eutectic reaction with a Mg₂Si phase due to heatgeneration during the extrusion process, causing local melting. If localmelting continuously occurs to couple melted parts, the material on thesurface of the extruded material will fall off due to the shearing forcethat the extruded material receives when passing through the die-bearingportion, causing burning of the extruded material.

Although the inner peripheral surface of the die is mirror-finished, theoccurrence of burning might coarsen the surface of the extrudedmaterial, losing the smoothness thereof.

The burning generated by the eutectic reaction in the Si, Al, and Mg₂Siphases can be reduced by applying the homogenization treatment to thebillet before extruding, at a temperature of 500 to 550° C. for fourhours or more, and separating (spheroidizing) the Si phase crystallizedin the strip shape.

On the other hand, the burning generated by the peritectic reactionbetween the β-AlFeSi and Mg₂Si phases can be reduced by conductinghomogenization treatment at a temperature of 500° C. or higher for along time (approximately 50 hours when Si and Fe contents are large),thereby converting the β-AlFeSi phase into a phase (spherodizing), or bydecreasing the extrusion rate to reduce the amount of heat generatedduring processing. However, the long-term homogenization treatmentinhibits productivity and is disadvantageous in terms of cost. Further,the reduction in the extrusion rate also inhibits productivity.

The present invention has been made in view of the foregoing problemsassociated with manufacturing of an Al—Mg—Si based aluminum alloyextruded material for machining, and it is an object of the presentinvention to obtain an Al—Mg—Si based aluminum alloy extruded materialwith a smooth surface and no burning without requiring the long-termhomogenization treatment and the reduction in extrusion rate.

Means for Solving the Problems

An Al—Mg—Si based aluminum alloy extruded material according to thepresent invention includes: Si: 2.0 to 6.0% by mass; Mg: 0.3 to 1.2% bymass; and Ti: 0.01 to 0.2% by mass, a Fe content being restricted to0.2% or less by mass, with the balance being Al and inevitableimpurities, wherein the number of AlFeSi particles having a diameter of5 μm or more is 20 or less per 50 μm square area of the extrudedmaterial, and the number of Mg₂Si particles having a diameter of 2 μm ormore is 20 or less per 50 μm square area of the extruded material, andwherein a ten-point average roughness Rz of a surface of the extrudedmaterial is 80 μm or less. The aluminum alloy extruded material canfurther contain one or more kinds of: Mn: 0.1 to 1.0% by mass; and Cu:0.1 to 0.4% by mass, as needed. The aluminum alloy extruded material canfurther contain one or more kinds of: Cr: 0.03 to 0.1% by mass; and Zr:0.03 to 0.1% by mass, as needed.

A method for manufacturing an Al—Mg—Si based aluminum alloy extrudedmaterial according to the present invention includes the steps of:applying a homogenization treatment to an aluminum alloy billet havingthe above-mentioned composition by keeping at 500 to 550° C. for 4 to 15hours; forcibly cooling the billet to 250° C. or lower at an averagecooling rate of 50° C./hr or higher; hot-extruding the billet at anextrusion rate of 3 to 10 m/min by heating at 450 to 500° C.; forciblycooling the extruded material at an average cooling rate of 50° C./secor higher; and applying an aging treatment to the extruded material. Bythis manufacturing method, the above-mentioned Al—Mg—Si based aluminumalloy extruded material according to the present invention can beobtained.

Effects of the Invention

Accordingly, the present invention can obtain the Al—Si—Mg basedaluminum alloy extruded material with excellent machinability and asmooth surface having a ten-point average roughness Rz of 80 μm or less,while reducing burning without being accompanied by the long-termhomogenization treatment as well as reduction in extrusion rate in themanufacture of the Al—Si—Mg based aluminum alloy extruded materialhaving a relatively large Si content.

The Al—Si—Mg based aluminum alloy extruded material in the presentinvention has high strength, excellent machinability, and goodappearance due to the smooth surface. Thus, the Al—Si—Mg based aluminumalloy extruded material enables reduction in amount of machiningprocessing, and can, in some cases, have a part of its surface used as asurface of a product as it is (without machining).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a scanning electron micrograph of a billet before ahomogenization treatment.

FIG. 2A shows a scanning electron micrograph of a billet in Example No.1 after the homogenization treatment.

FIG. 2B shows a scanning electron micrograph of an extruded materialobtained from the billet in Example No. 1.

FIG. 3A shows a scanning electron micrograph of a billet in Example No.12 after the homogenization treatment.

FIG. 3B shows a scanning electron micrograph of an extruded materialobtained from the billet in Example No. 12.

FIG. 4A shows a scanning electron micrograph of a billet in Example No.13 after the homogenization treatment.

FIG. 4B shows a scanning electron micrograph of an extruded materialobtained from the billet in Example No. 13.

BEST MODE FOR CARRYING OUT THE INVENTION

An Al—Si—Mg based aluminum alloy extruded material and a manufacturingmethod therefor according to the present invention will be described inmore detail below.

(Aluminum Alloy Composition)

An aluminum alloy in the present invention includes: Si: 2.0 to 6.0% bymass; Mg: 0.3 to 1.2% by mass; and Ti: 0.01 to 0.2% by mass, with thebalance being Al and inevitable impurities. The aluminum alloy furtherincludes one or more kinds of: Mn: 0.1 to 1.0% by mass; and Cu: 0.1 to0.4% by mass, as needed. Moreover, the aluminum alloy includes one ormore kinds of: Cr: 0.03 to 0.1% by mass; and Zr: 0.03 to 0.1% by mass,as needed. Although the aluminum alloy compositions itself is wellknown, the present invention is characterized by that a Fe content inthe inevitable impurities is restricted to 0.2% or less by mass. Eachcomponent of the aluminum alloy in the present invention will bedescribed below.

Si: 2.0 to 6.0% by Mass

Silicon (Si) serves to form Si-based crystallized grains (Si phase) inaluminum, which are second-phase hard particles, and to improve thefragmentation of chips and the machinability. To this end, Si needs tobe added in an amount of 2% or more by mass that exceeds the amount ofsolid solution of Si into aluminum. On the other hand, the addition ofmore than 6% by mass of Si might form coarsened Si phases, whereby theeutectic reaction among the Si phase, the Al phase, and Mg₂Si phaseoccur to lower the melting start point. To prevent the occurrence oflocal melting and burning together with a decrease in melting startpoint, it is necessary to suppress the amount of heat during anextrusion process. For this reason, the extrusion rate needs to bereduced. Therefore, the Si content is set at 2.0 to 6.0% by mass. Thelower limit of Si content is preferably 3.5% by mass, while the upperlimit of Si content is preferably 4.5% by mass.

Mg: 0.3 to 1.2% by Mass

Magnesium (Mg) precipitates fine particles of Mg₂Si by the agingprecipitation treatment, thereby improving the strength of the extrudedmaterial. Thus, Mg is desirably added in an amount of 0.3% or more bymass. On the other hand, Mg₂Si is also formed as crystallized grainsduring solidification and might cause peritectic reaction with β-AlFeSiduring the extrusion, which leads to local melting, causing burning ofthe extruded material. When the Mg content exceeds 1.2% by mass, thecrystallized grains of Mg₂Si are formed in a large amount, so that theburning of the extruded material tends to easily occur. Therefore, theMg content is set at 0.3 to 1.2% by mass. The lower limit of Mg contentis preferably 0.5% by mass, while the upper limit of Mg content ispreferably 0.9% by mass.

Ti: 0.01 to 0.2% by Mass

Titanium (Ti) serves to refine a cast structure, thereby stabilizing themechanical properties of the extruded material. To attain this effect,Ti is added. However, when the Ti content is less than 0.01% by mass,its effect cannot be obtained. On the other hand, even if the Ti contentexceeds 0.2% by mass, the effect of refining cannot be further improved.Therefore, the Ti content is set at 0.01 to 0.2% by mass. The lowerlimit of Ti content is preferably 0.01% by mass, while the upper limitof Ti content is preferably 0.1% by mass.

Mn: 0.1 to 1.0% by Mass; and Cu: 0.1 to 0.4% by Mass

Manganese (Mn) has an effect of improving the strength of extrudedmaterial by being precipitated as dispersion particles during thehomogenization treatment to thereby refine crystal grains of theextruded material. For this reason, Mn is added as needed. When the Mncontent is less than 0.1% by mass, the above-mentioned effect cannot besufficiently exhibited. On the other hand, when the Mn content exceeds1.0% by mass in adding Mn, the extrudability is degraded. Therefore, theMn content is set at 0.1 to 1.0% by mass. The lower limit of Mn contentis preferably 0.4% by mass, while the upper limit of Mn content ispreferably 0.8% by mass.

Copper (Cu) is added as appropriate, instead of or together with Mn, toenhance the strength of the extruded material by being solid-soluted.When the Cu content is less than 0.1% by mass, the above-mentionedeffect cannot be sufficiently exhibited. On the other hand, when the Cucontent exceeds 0.4% by mass in adding Cu, the corrosion resistance andextrudability of the extruded material are degraded. Therefore, the Cucontent is set at 0.1 to 0.4% by mass. The lower limit of Cu content ispreferably 0.2% by mass, while the upper limit of Cu content ispreferably 0.3% by mass.

Cr: 0.03 to 0.1% by Mass; and Zr: 0.03 to 0.1% by Mass

Chrome (Cr) is added as appropriate to suppress recrystallization andrefine crystal grains, thereby enhancing the strength of the extrudedmaterial. However, when the Cr content is less than 0.03% by mass, theabove-mentioned effect cannot be sufficiently obtained. On the otherhand, when the Cr content exceeds 0.1% by mass in adding Cr, burningtends to occur in the extruded material during the extrusion process.Therefore, the Cr content is set at 0.03 to 0.1% by mass.

Zinc (Zr) is added as appropriate, instead of or together with Cr, tosuppress the recrystallization and refine crystal particles, therebyenhancing the strength of the extrusion. However, when the Zr content isless than 0.03% by mass, the above-mentioned effect cannot besufficiently obtained. On the other hand, when the Zr content exceeds0.1% by mass in adding Zr, a compound including a mixture of Al and Zrbecomes coarsened during the homogenization treatment, failing toexhibit the effect of suppressing the recrystallization. Therefore, theZr content is set at 0.03 to 0.1% by mass.

Fe: 0.2% or Less by Mass

Iron (Fe) existing as the inevitable impurity in the aluminum alloygenerates β-AlFeSi phase, which is a needle-like crystallized grain,during a cooling process after casting. To reduce the β-AlFeSi contentin the billet and to prevent burning during the extrusion process, thehomogenization treatment needs to be performed to convert the β-AlFeSiphase into a phase (spherodizing), or the Fe content in the aluminumalloy needs to be decreased.

However, to convert the β-AlFeSi phase into a phase, the homogenizationtreatment is required to be carried out at a high temperature for a longtime, which degrades the productivity of the extrusions. In contrast,when the Fe content of the aluminum alloy is restricted to 0.2% or lessby mass, the amount of the generated β-AlFeSi phases is reduced. Amanufacturing method to be described below can prevent the burning ofthe extrusions during the extrusion process without applying thehomogenization treatment for a long time. Note that the Fe contentnormally included as the inevitable impurity in the aluminum alloy isapproximately 0.3% by mass.

(Method for Manufacturing Aluminum Alloy Extruded Material) HomogeneousTreatment Conditions

The homogenization treatment for the cast billet is performed underholding conditions of 500 to 550° C. for 4 to 15 hours. A holdingtemperature is set at 500° C. or higher, and a holding time is set at 4hours or more. This is because strip-shaped crystallized Si phases aredivided (spheroidized) while crystallized Mg₂Si is solid-soluted. As theholding temperature is higher and the holding time is longer, theseconditions would be more preferable for the homogenization treatmentbecause they promote the division of Si phase and the solid solution ofMg₂Si and reduce the burning. However, at a temperature exceeding 550°C., local dissolution might occur, while for a holding time exceeding 15hours, the productivity of extrusions might be reduced. Therefore, thehomogenization treatment should be performed under holding conditions,specifically, at a temperature of 500 to 550° C. and for a time of 4 to15 hours. Note that these holding conditions cannot sufficiently achievethe conversion of β-AlFeSi phase into a phase.

Cooling Conditions after Homogenization Treatment

After the homogenization treatment, the billet is forcibly cooled at anaverage cooling rate of 50° C./hr or higher. Conventionally, the billetobtained after the homogenization treatment is taken out of a furnaceand cooled by being allowed to stand, or by being air-cooled. In thereal operation, since a number of high-temperature billets are cooledwhile being accumulated, the cooling rate is generally estimated to beless than 30° C./hr even in air-cooling with fans. No attention has beenpaid particularly to the cooling rate after the homogenizationtreatment. At an average cooling rate of 50° C./hr or higher, the billetis forcibly cooled to a temperature of less than 250° C., which canminimize the precipitation of Mg₂Si (to such a degree that can preventthe occurrence of burning during extrusion). At a temperature of 250° C.or lower, the billet may be allowed to stand to cool to the roomtemperature. The desirable average cooling rate is 80° C./hr or more,which can be achieved by forcibly performing air-cooling with fans underthe condition that the billets are not accumulated. Further, watercooling is more desirable. In this case, the cooling rate of about100,000° C./hr can be achieved.

Extrusion Conditions

After the homogenization treatment, the billet is reheated to atemperature of 450 to 500° C. and then subjected to hot-extruding at anextrusion rate of 3 to 10 m/min. Since the extruded material in thepresent invention is a solid-core material (solid material), theextrusion ratio thereof is relatively small, and the heat generationtherefrom does not become so much during processing. At an extrusiontemperature of less than 450° C., the temperature of the extrudedmaterial at the outlet of an extrusion machine does not reach 500° C. orhigher that is required for solution. On the other hand, once theextrusion temperature exceeds 500° C., the processing heat generation isadded, increasing the temperature of extrusion material, leading to therisk of burning of the extruded material. Therefore, the extrusiontemperature (heating temperature of the billet) is set at 450 to 500° C.At an extrusion rate of less than 3 m/min, the productivity of extrudedmaterials is degraded. On the other hand, once the extrusion rateexceeds 10 m/min, the processing heat generation becomes large,increasing the temperature of material for extrusion, leading to therisk of burning of the extruded material. When the extruded material hasa corner at its cross section, the phenomenon of corner cracks tends tooccur as metal does not spread out into the corner. Thus, the extrusionrate is set at 3 to 10 m/min. In the manufacturing method of the presentinvention, the extrusion ratio (i.e. the ratio of the cross-sectionalarea of an extrusion container to that of the extrusion outlet) ispreferably in a range of 15 to 40.

Cooling Conditions after Extrusion

The extruded material obtained directly after the extrusion process isforcibly cooled (die-quenched) online from the outlet temperature of theextrusion machine to a temperature of 250° C. or less at an averagecooling rate of 50° C./sec or higher. At a temperature of 250° C. orlower, the extruded material may be allowed to stand to cool to the roomtemperature. By setting the average cooling rate to 50° C./sec orhigher, the precipitation of Mg₂Si is prevented. Preferable coolingmeans is water-cooling.

Aging Treatment Conditions

The extruded material die-quenched is subjected to an aging treatment.The aging treatment may be performed at a temperature of 160 to 200° C.for 2 to 10 hours.

(Number Density of AlFeSi Particles and Mg₂Si Particles in ExtrudedMaterial)

The distribution state of the coarse β-AlFeSi particles and Mg₂Siparticles in the Al—Mg—Si based aluminum alloy extruded material in thepresent invention reflects the distribution state of the β-AlFeSi phaseand Mg₂Si phase in the billet after the homogenization treatment (aftercooling). This point will be described referring to scanning electronmicrographs of FIGS. 2A to 4B.

FIGS. 2A, 3A, and 4A are the scanning electron micrographs showing thedistribution states of β-AlFeSi phases and Mg₂Si phases in the billetsof Examples No. 1, 12, and 13, respectively. The β-AlFeSi phase is shownas white needle-like particles, and the Mg₂Si phase is shown as blackgranular particles. FIGS. 2B, 3B, and 4B are the scanning electronmicrographs showing the distribution states of AlFeSi particles andMg₂Si particles in the extrusion material obtained from these billets,respectively. The original β-AlFeSi phase is divided when being extrudedand then formed into an aggregate of white granular particles.

As shown in Table 2 of Examples to be mentioned later, referring to FIG.2B, each of the number of AlFeSi particles having a diameter of 5 μm ormore and the number of Mg₂Si particles having a diameter of 2 μm or moreper certain area (50 μm square) falls within a specified range of thepresent invention. Using the distribution state of each kind ofparticles shown in FIG. 2B as a reference, as illustrated in FIG. 3B,the number of AlFeSi particles having a diameter of 5 μm or more isrelatively large, exceeding the specified range of the presentinvention, while as illustrated in FIG. 4B, the number of Mg₂Siparticles having a diameter of 2 μm or more is relatively large,exceeding the specified range of the present invention. By comparisonbetween the distribution state of β-AlFeSi phases and that of Mg₂Siphases with reference to FIGS. 2A, 3A, and 4A, as illustrated in FIG.2A, the amount of β-AlFeSi phases is small while the size of Mg₂Siphases is small; as illustrated in FIG. 3A, the amount of β-AlFeSiphases is relatively large; and as illustrated in FIG. 4A, the size ofMg₂Si phases is relatively large.

In this way, when the number of coarse AlFeSi particles having adiameter of 5 μm or more in the extruded material is large, it suggeststhat the amount of the β-AlFeSi phases in the billet before theextrusion (after the homogenization treatment) is large. When the numberof coarse Mg₂Si particles having a diameter of 2 μm or more in theextruded material is large, it suggests that the size of the Mg₂Siparticles in the billet before the extrusion (after the homogenizationtreatment) is large. These relationships can be satisfied except forwhen the extrusion ratio is excessively large (e.g., 45 or higher).Thus, the distribution states of the β-AlFeSi particles having adiameter of 5 μm or more and of the Mg₂Si particles having a diameter of2 μm or more in the extruded material are specified, thereby indirectlyspecifying the distribution states of the β-AlFeSi phases and Mg₂Siphases in the billet before the extrusion (after the homogenizationtreatment).

When the numbers of AlFeSi particles having a diameter of or more andMg₂Si particles having a diameter of 2 μm or more per certain area inthe extruded material are within respective specific ranges in thepresent invention, the amount of generated β-AlFeSi phases in the billetis small, the precipitation of Mg₂Si particles in the billet issuppressed, and the size of Mg₂Si phase is small. Conversely, when thenumber of AlFeSi particles having a diameter of 5 μm or more per certainarea in the extruded material exceeds the specific range in the presentinvention, the amount of generated β-AlFeSi phases in the billet islarge. When the number of Mg₂Si particles having a diameter of 2 μm ormore per certain area in the extruded material exceeds the specificrange in the present invention, the precipitation of Mg₂Si phase in thebillet is not sufficiently suppressed and the size of Mg₂Si phase in thebillet is large.

The number densities of the AlFeSi particles and Mg₂Si particles in thepresent invention will be measured in the following procedure.

1) After grinding the cross section of the extruded material to have itsnumber density measured, two or more observation regions in 50 μm square(a pair of sides being in parallel to the extrusion direction) where thenumber density is measured are selected from the cross section byobservation with a scanning electron microscope (SEM).

2) The numbers of AlFeSi particles having a diameter of 5 μm or more andof Mg₂Si particles having a diameter of 2 μm or more that are includedin these observation regions are respectively measured (the diameter ofeach particle being the circle equivalent diameter). Note that toachieve the accurate measurements, the magnification scale of SEM ispreferably set at 1,000 times or more in measuring the number ofparticles included in the region. The particles existing on the side ofthe observation region is counted as one.

3) The number of each kind of particles is measured for each selectedobservation region in the above-mentioned procedure 2), and an averagevalue of the numbers of each kind of particles in all selectedobservation regions is determined.

(Surface Roughness of Extruded Material)

The billet of the Al—Mg—Si based aluminum alloy with the above-mentionedcomposition is subjected to the homogenization treatment under theconditioned mentioned above, so that the strip-shaped Si phasescrystallized in the billet are spheroidized and Mg₂Si is solid-saluted.Subsequently, the billet held at the homogeneous processing temperatureis forcibly cooled to 250° C. or lower at a cooling rate of 50° C./hr ormore, which is larger than the usual one, thereby suppressing theprecipitation of Mg₂Si particles during the cooling process. Since thebillet is designed to reduce the amount of generated β-AlFeSi phases andto suppress the precipitation of Mg₂Si phases, the peritectic reactionbetween the β-AlFeSi phase and Mg₂Si phase is suppressed, and theprecipitation of Mg₂Si phase is suppressed during the extrusion process,whereby the eutectic reaction among Si, Al, and Mg₂Si is alsosuppressed. As a result, an Al—Mg—Si based aluminum alloy extrudedmaterial (extruded material as it is) can be manufactured that reducesburning of the extruded material and has a small surface roughness. Inthe present invention, the surface roughness of the Al—Mg—Si basedaluminum alloy extruded material can be reduced to 80 μm or lower interms of ten-point average roughness Rz (JIS B0601:1994).

EXAMPLES

An Al—Si—Mg based aluminum alloy having a chemical composition shown inTable 1 (composition after fusion) was fused and then subjected tosemicontinuous casting, thereby producing a billet having a diameter of400 mm. The billet was subjected to the homogenization treatment underthe homogenization treatment conditions (holding temperature, holdingtime and cooling rate) shown in Table 1. Note that the balance of thecomposition mentioned in Table 1 included Al and inevitable impuritiesexcept for Fe. Subsequently, extrusion molding was performed on thebillet at an extrusion ratio of 33 under the extrusion conditions shownin Table 1 (extrusion temperature (billet heating temperature),extrusion rate and cooling rate), thereby producing a solid extrudedmaterial having a rectangular cross section (100 mm×40 mm), followed byan aging treatment at 180° C. for 4 hours. Note that the term “coolingrate” in each case means a cooling rate to 250° C.

TABLE 1 Extrusion condition Homogeneous treatment conditions CoolingComposition (% by mass) Temperature Cooling rate Temperature Rate rateNo. Si Fe Mg Cu Mn Ti Cr Zr ° C. Time h ° C./h ° C. m/min ° C./s 1 4.020.15 0.74 Tr. 0.64 0.02 Tr. Tr. 520 14 80 475 4.5 100 2 5.81 0.16 0.34Tr. 0.32 0.02 Tr. Tr. 500 14 80 479 5.0 50 3 2.15 0.14 0.46 Tr. 0.950.02 Tr. Tr. 520 14 80 480 5.5 80 4 5.64 0.20 0.57 Tr. 0.59 0.03 Tr. Tr.500  5 50 476 7.5 100 5 5.86 0.15 0.75 0.23 0.41 0.03 Tr. Tr. 520  5 50471 8.5 120 6 3.72 0.13 0.74 0.36 Tr. 0.19 Tr. Tr. 520  5 50 470 8.5 2007 4.53 0.05 0.45 0.38 0.15 0.04 0.03 Tr. 520  5 50 473 5.0 150 8 3.650.08 0.35 Tr. Tr. 0.04 Tr. 0.08 520 14 80 490 3.0 130 9 3.54 0.14 0.84Tr. Tr. 0.06 0.03 Tr. 500 14 50 480 5.0 100 10 6.34* 0.16 0.86 0.25 Tr.0.02 0.03 Tr. 520 14 120  470 3.0 100 11 1.54* 0.13 0.64 Tr. 0.36 0.020.03 Tr. 520 14 120  470 3.0 100 12 5.75 0.22* 0.85 0.23 Tr. 0.02 0.03Tr. 520 14 120  470 3.0 100 13 4.62 0.16 0.74 Tr. 0.63 0.02 Tr. Tr. 52014  40* 470 3.0 100 14 4.10 0.14 0.69 Tr. 0.62 0.02 Tr. Tr. 520  3* 80470 3.0 100 15 3.60 0.17 0.54 Tr. 0.61 0.02 Tr. Tr.  480* 14 120  4703.0 100 16 3.72 0.26* 0.62 0.21 Tr. 0.02 0.03 Tr. 520 14 120  470  2.5*100 17 4.46 0.23* 0.70 0.23 Tr. 0.02 0.04 Tr. 520  22* 120  470 3.5 10018 5.64 0.14 0.90 Tr. 0.58 0.02 Tr. Tr. 520 14 80  520* 3.5 100 19 2.980.05 1.02 Tr. 0.57 0.02 Tr. Tr. 520 14 80 470 12.0* 100 20 3.56 0.160.65  0.56* Tr. 0.06 Tr. Tr. 500 15 50 480 5.0 90 21 3.67 0.15 0.20*0.32 Tr. 0.06 Tr. 0.05 520 10 60 475 3.0 120 22 4.32 0.10 1.25* Tr. 0.450.05 Tr. Tr. 515  6 100  480 3.0 130 *Item departing from specific rangeof the present invention

The thus-obtained extruded material was used as a sample material, andeach sample material was measured on the number densities of coarseAlFeSi particles and Mg₂Si particles, machinability, hardness, surfaceroughness (ten-point average roughness Rz), and extrudability in thefollowing way.

(Number Densities of AlFeSi Particles and Mg₂Si Particles)

After grinding the cross section of each sample material to have itsnumber density measured, two square observation regions in 50 μm square(a pair of sides being in parallel to the extrusion direction) formeasurement of the number density were selected from each samplematerial by observation with a scanning electron microscope (SEM). Foreach sample materials, the two selected observation regions wereobserved with the SEM having a magnification scale set at 1,000 times,and then the number of AlFeSi particles having a diameter (circleequivalent diameter) of 5 μm or more and the number of Mg₂Si particleshaving a diameter (circle equivalent diameter) of 2 μm or more thatcould be observed in each observation range were measured. The averageof the number of each kind of particles measured in the two observationregions was determined. The results of the measurements are shown inTable 2. Note that the particles existing on the side of the observationregion was counted as one.

(Machinability)

A hole punching was performed on each sample using a commerciallyavailable high-speed steel drill having a diameter of 4 mm under theconditions, specifically, at the number of revolutions of 1500 rpm and afeeding velocity of 300 mm/min, and then the number of machining chipsin 100 g machining chip aggregate obtained was counted to measure themachinability of the extruded material in each sample (fragmentation ofmachining chip). Samples containing more than 7000 machining chips arerated excellent “A”; samples containing 5000 to 7000 machining chips arerated good “B”; samples containing 3000 to less than 5000 machiningchips are rated satisfactory “C”; and samples containing less than 3000machining chips are rated unsatisfactory “D”. The results of themeasurements are shown in the item “properties” of Table 2.

(Hardness)

A Rockwall hardness (HRB) of each sample was measured based on theRockwell hardness test of JIS Z 2245:2011 as a test method.

(Surface Roughness)

The upper, lower, left and right surfaces (four surfaces in total) ofthe extruded material in each sample were visually observed across itsentire length. The surface roughness (ten-point average roughness Rz) ofa part of each surface, at which its surface roughness was determined tobe largest by the visually observation, was measured in the directionvertical to the extrusion direction based on the standard of JISB0601:1994. The maximum ten-point average roughness Rz obtained at eachsurface is shown as the surface roughness (ten-point average roughnessRz) of the extruded material in the item “properties” of Table 2.

(Extrudability)

The corners of the extruded materials in samples Nos. 1 to 22 werevisually observed across the entire length of the extruded material, andthe presence or absence of occurrence of any corner crack (whether theextrudability were good or bad) was checked for each sample.Additionally, regarding the billets corresponding to the extrudedmaterials in specimen Nos. that were observed to have any corner crack,each billet was extruded at an extrusion rate lower than the extrusionrate shown in Table 1, and then the presence or absence of occurrence ofthe corner crack was checked. Further, regarding the billetscorresponding to the extruded materials in specimen Nos. observed tohave no corner crack, the billet was extruded at an extrusion ratehigher than the extrusion rate shown in Table 1, and then the presenceor absence of occurrence of the corner crack was checked. At this time,the extrusion rates were set at any one of 3 m/min, 5 m/min, and 10m/min, and the homogenization treatment conditions and the extrusionconditions (except for the extrusion rate) were set as mentioned inTable 1. Samples that were observed to have no corner crack at theextrusion rate of 10 m/min were rated as having excellent extrudability“A”; samples that were observed to have a corner crack at the extrusionrate of 10 m/min but no corner crack at the extrusion rate of 5 m/minwere rated as having good extrudability “B”; and samples that wereobserved to have a corner crack even at the extrusion rate of 3 m/minwere rated as having bad extrudability “C”. The results of themeasurements are shown in Table 2.

TABLE 2 Number density Properties of particles Hardness RZ No. AlFeSiMg₂Si Machinability HRB μm Extrudability 1 12 12 B 50.6 56 A 2 15  6 A46.8 64 B 3 14  8 C 50.6 19 A 4 18 12 A 53.0 78 B 5 15 16 A 56.2 70 B 610 19 B 54.7 53 A 7  5 14 B 56.4 50 A 8  4 12 B 45.6 50 A 9 11 16 B 38.5  55.8 A 10 19 17 A 60.5  97* B 11  9 15 D 50.5   53.1 A 12  27* 16 B56.4   85.4* B 13 18  27* B 54.1   90.6* A 14  22*  21* B 53.0 102* A 15 23* 20 B 54.2 105* A 16  21* 16 B 56.1   74.1 A 17  22* 20 B 52.8  72.8 A 18 19 15 B 57.0 132* B 19  4 15 B 59.4 142* A 20 12 11 B 59.4  45.6 C 21 13  4 B 35.2*   56.7 A 22  9  26* B 65.0   87.9* A *Itemdeparting from specific range of the present invention

As shown in Tables 1 and 2, the extruded materials Nos. 1 to 9 had thecomposition specified by the present invention and satisfied the numberdensities of the AlFeSi particles and Mg₂Si particles, whereby theseextruded materials had a small surface roughness (ten-point averagesurface roughness Rz≤80 μm) and excellent machinability. Further, theseextruded materials had a Rockwell hardness of 38 HRB or more andexcellent strength. The extruded materials Nos. 1 to 9 were manufacturedby the manufacturing method specified by the present invention. FIGS. 2Aand 2B illustrate the scanning electron micrographs of the billet No. 1(after the homogenous treatment) and the extruded material obtained fromthe billet No. 1, respectively.

On the other hand, the extruded material No. 10 had burning occurredbecause of the excessive Si content and had the large surface roughness.

The extruded material No. 11 had degraded machinability because ofexcessively small Si content.

In the extruded material No. 12, the number density of AlFeSi particlesexceeded the specific range of the present invention because of theexcessive content of Fe as inevitable impurity, resulting in largesurface toughness (ten-point average toughness Rz>80 μm). FIGS. 3A and3B illustrate the scanning electron micrographs of the billet No. 12(after the homogenous treatment) and the extruded material obtained fromthe billet No. 12, respectively. As shown in FIG. 3A, the amount ofβ-AlFeSi phases is large in the billet, causing burning during theextrusion process, resulting in the large surface roughness.

In the extruded material of sample No. 13, the number density of Mg₂Siparticles exceeded the specific range of the present invention,resulting in large surface roughness (ten-point average roughness Rz>80μm). FIGS. 4A and 4B illustrate the scanning electron micrographs of thebillet No. 13 (after the homogenous treatment) and the extruded materialobtained from the billet No. 13, respectively. As shown in FIG. 4A,since the cooling rate after the homogenization treatment is low, thesize of the Mg₂Si phase in the billet becomes large, causing burningduring the extrusion process, resulting in a large surface roughness.

In the extruded material of sample No. 14, the number densities ofAlFeSi particles and Mg₂Si particles exceeded the respective specificrange of the present invention, and in the extruded material of sampleNo. 15, the number density of AlFeSi particles exceeded the specificrange of the present invention, resulting in large surface roughness inboth sample No. 14 and No. 15 (ten-point average roughness Rz>80 μm).This is because in sample No. 14, the time for the homogenizationtreatment was short, while in sample No. 15, the temperature of thehomogenization treatment was low, whereby in both samples, theconversion of the β-AlFeSi particles into a phase did not proceed, andthe division of the Si phase as well as the solid-solution of Mg₂Siphase in the billet were insufficient.

In both the extruded materials in samples No. 16 and 17, the Fe contentwas excessive, and the number density of AlFeSi particles exceeded thespecific range of the present invention, but the surface roughness wassmall (ten-point average roughness Rz≤80 μm). This is because in sampleNo. 16, the extrusion rate was set much lower than the lower limit ofthe specific range, namely, 3 m/min, while in sample No. 17, the timefor the homogenization treatment was set much longer than the upperlimit of the specific range, namely, 15 hours. In this way, theproductivity in each of samples No. 16 and 17 was degraded.

In the extruded materials of samples No. 18 and 19, both the numberdensities of AlFeSi particles and Mg₂Si particles satisfied the specificranges of the present invention, but their surface roughness were large(ten-point average roughness Rz>80 μm). This is because in sample No.18, the extrusion temperature was too high, while in sample No. 19, theextrusion rate was too high, increasing the material temperature due tothe heat generation during the processing, causing burning in theextruded material.

In the extruded material of sample No. 20, the Cu content was excessive,and thus the extrudability were degraded.

In the extruded material of sample No. 21, the Mg content was too small,and thus the strength (hardness) thereof was low.

In the extruded material of sample No. 22, the Mg content was excessive,and the number density of Mg₂Si particles exceeded the specific range ofthe present invention, resulting in a large surface roughness (ten-pointaverage roughness Rz>80 μm). This is considered to be because Mg₂Siphases are formed in a large amount in the billet due to the excessiveMg content, causing burning in the extruded material during theextrusion process.

The present application claims priority based on Japanese PatentApplication No. 2013-177572 filed on Aug. 29, 2013 and Japanese PatentApplication No. 2014-156634 filed on Jul. 31, 2014, the disclosures ofall of which are incorporated into the present specification byreference.

1. An Al—Si—Mg based aluminum alloy extruded material comprising: Si:2.0 to 6.0% by mass; Mg: 0.3 to 1.2% by mass; and Ti: 0.01 to 0.2% bymass, a Fe content being restricted to 0.2% or less by mass, with thebalance being Al and inevitable impurities, wherein the number of AlFeSiparticles having a diameter of 5 μm or more is 20 or less per 50 μmsquare area of the extruded material, the number of Mg₂Si particleshaving a diameter of 2 μm or more is 20 or less per 50 μm square area ofthe extruded material, and a ten-point average roughness Rz of a surfaceof the extruded material is 80 μm or less.
 2. The Al—Si—Mg basedaluminum alloy extruded material according to claim 1, furthercomprising one or more kinds of: Mn: 0.1 to 1.0% by mass; and Cu: 0.1 to0.4% by mass.
 3. The Al—Si—Mg based aluminum alloy extruded materialaccording to claim 1, further comprising one or more kinds of: Cr: 0.03to 0.1% by mass; and Zr: 0.03 to 0.1% by mass.
 4. The Al—Si—Mg basedaluminum alloy extruded material according to claim 2, furthercomprising one or more kinds of: Cr: 0.03 to 0.1% by mass; and Zr: 0.03to 0.1% by mass. 5-8. (canceled)